removed nimKeepAlive as it's not necessary with interior pointer checking

lib/system/gc.nim

@@ -34,8 +34,6 @@ const
   rcWhite = 0b010  # member of a garbage cycle
   rcPurple = 0b011 # possible root of a cycle
   rcZct = 0b100    # in ZCT
-  rcMarked = 0b101 # dummy write to keep C code generator from
-                   # eliminating the root
   rcRed = 0b101    # Candidate cycle undergoing sigma-computation
   rcOrange = 0b110 # Candidate cycle awaiting epoch boundary
   rcShift = 3      # shift by rcShift to get the reference counter
@@ -588,11 +586,6 @@ proc gcMark(gch: var TGcHeap, p: pointer) {.inline.} =
         add(gch.decStack, cell)
   sysAssert(allocInv(gch.region), "gcMark end")
 
-proc nimKeepAlive(p: PGenericSeq) {.compilerRtl, noinline.} =
-  var c = usrToCell(p)
-  if isAllocatedPtr(gch.region, c):
-    c.refcount = c.refcount or rcMarked
-
 proc markThreadStacks(gch: var TGcHeap) = 
   when hasThreadSupport and hasSharedHeap:
     {.error: "not fully implemented".}